1. Field of the Invention
This invention relates generally to direct oxidation fuel cells, and more particularly, to diffusion layers for such fuel cells.
2. Background Information
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suited for use as a fuel depending upon the materials chosen for the components of the cell. Organic materials, such as methanol or natural gas, are attractive choices for fuel due to the their high specific energy.
Fuel cell systems may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion to extract hydrogen from the fuel before it is introduced into the fuel cell system) or “direct oxidation” systems in which the fuel is fed directly into the cell without the need for separate internal or external processing. Most currently available fuel cells are reformer-based fuel cell systems. However, because fuel processing is expensive and requires significant volume, reformer based systems are presently limited to comparatively high power applications.
Direct oxidation fuel cell systems may be better suited for a number of applications in smaller mobile devices (e.g., mobile phones, handheld and laptop computers), as well as in some larger applications. Typically, in direct oxidation fuel cells, a carbonaceous liquid fuel in an aqueous solution (typically aqueous methanol) is applied to the anode face of a membrane electrode assembly (MEA). The MEA contains a protonically-conductive but, electronically non-conductive membrane (PCM). Typically, a catalyst which enables direct oxidation of the fuel on the anode is disposed on one surface of the PCM (or is otherwise present in the anode chamber of the fuel cell). Protons (from hydrogen found in the fuel and water molecules involved in the anodic reaction) are separated from the electrons. The protons migrate through the PCM, which is impermeable to the electrons. The electrons thus seek a different path to reunite with the protons and oxygen molecules involved in the cathodic reaction and travel through a load, providing electrical power.
One example of a direct oxidation fuel cell system is a direct methanol fuel cell system or DMFC system. In a DMFC system, methanol in an aqueous solution is used as fuel (the “fuel mixture”), and oxygen, preferably from ambient air, is used as the oxidizing agent. There are two fundamental reactions that occur in a DMFC which allow a DMFC system to provide electricity to power consuming devices: the anodic disassociation of the methanol and water fuel mixture into CO2, protons, and electrons; and the cathodic combination of protons, electrons and oxygen into water. The overall reaction may be limited by the failure of either of these reactions to proceed to completion at an acceptable rate (more specifically, failure to oxidize the fuel mixture will limit the cathodic generation of water, and vice versa).
Typical DMFC systems include a fuel source, fluid and effluent management systems, and a direct methanol fuel cell (“fuel cell”). The fuel cell typically consists of a housing, and a membrane electrode assembly (“MEA”) disposed within the housing. A typical MEA includes a centrally disposed protonically-conductive, electronically non-conductive membrane (“PCM”). One example of a commercially available PCM is Nafion® a registered trademark of E.I. Dupont Nemours and Company, a cation exchange membrane based on perflouorocarbon polymers with side chain termini of perflourosulfonic acid groups, in a variety of thicknesses and equivalent weight. The PCM is typically coated on each face with an electrocatalyst such as platinum, or platinum/ruthenium mixtures or alloy particles. On either face of the catalyst coated PCM, the electrode assembly typically includes a diffusion layer.
A conventional diffusion layer serves to evenly distribute liquids and gases across the electrodes. In the case of the anode, the diffusion layer is used to evenly distribute the fuel/water mixture to a maximum number of contact points on the surface of the anode so that the greatest surface area of the anode is utilized for methanol electro-oxidation. On the cathode side, the diffusion layer disperses oxygen so that it is more evenly introduced to the cathode face of the PCM to promote the oxygen electro-reduction, which produces water. In addition, flow field plates are often placed on the surface of the diffusion layers, but are not usually in direct contact with the coated PCM. The flow field plates function to provide mass transport of reactants and byproducts of the electrochemical reactions, and the flow field plates may also have a current collection functionality, in that the flow field plates act to collect and conduct electrons to the load.
A typical diffusion layer may be fabricated of carbon paper or a carbon cloth, typically with a microporous coating made of a mixture of carbon powder and polytetrafluoroethylene (“PTFE), sold commercially as TEFLON®, a trademark of the E.I. DuPont de Nemours and Company. The PTFE component has a function of wet proofing in the case of a gas-supplied electrode, but as the cell reaction proceeds, the carbon paper or carbon cloth can become saturated with liquid water. This can be caused by continuous water build-up in the cathode chamber of the fuel cell. The cathode can become “flooded”, in which case the cathode half of the reaction can be compromised or even prevented. This results in the overall performance of the cell being compromised, or prevented.
Typically, the risk of cathode flooding is mitigated by active air flow to remove water from the cathode layer. This, however, increases the cost and complexity of the fuel cell system, thus adding to the expense of manufacture, as well as introducing the possibility of parasitic losses. In addition, it also adds volume to a system that must meet demanding form factors.
It is also noted that when water builds up in the cathode, it not only can cause flooding, which reduces the effectiveness of the half reaction on the cathode side, but it also results in pressure on the cathode face of the PCM that can weaken or compromise the bond between the membrane (PCM) and the catalytic coating, or the bond between the diffusion layer and the catalytic coating. Cell performance can be reduced over the long run because these stresses can ultimately cause separation of key fuel cell components, preventing the effective operation thereof.
There remains a need therefore for a diffusion layer that provides optimal gas diffusion properties, and resists flooding of the cathode portion of the fuel cell. In addition, in direct methanol fuel cells (DMFCs), the removal and collection of liquid water from the cathode by means of such a modified cathode diffusion layer is of high significance in maintaining overall water balance in the fuel cell system. Effective collection of liquid water at the cathode may be prerequisite to carrying just neat (pure) methanol, rather than methanol/water mixtures, as fuel supply to the DMFC.
It is thus an object of the invention to provide a diffusion layer that reduces the risk of cathode flooding, and liquid water-caused deterioration on the cathode side of the fuel cell and/or a cathode diffusion layer that allows liquid water to be collected for use in the direct methanol fuel cell system.